1. Field of the Invention
This disclosure relates to a polypropylene molded article and a method for manufacturing the polypropylene molded article.
2. Description of the Related Art
Commodity plastics such as polyethylene (PE), polypropylene (PP), polystyrene (PS), polyvinyl chloride (PVC), etc., are inexpensive and cost as little as 100 yen/kg or less. Commodity plastics are easy to mold and are light-weight since they weigh a fraction of what metals or ceramics weigh. Accordingly, commodity plastics are being widely used as materials for various household products including bags, various packaging, various containers, and sheets as well as materials for industrial parts such as auto parts and electric parts, daily commodities, sundry goods, etc.
However, commodity plastics have drawbacks such as insufficient mechanical strength. Under the current situation, commodity plastics do not have properties sufficient for materials used in various industrial products such as mechanical products, e.g., automobiles, and electric, electronic, and information-related products. The application range of commodity plastics has thus been limited.
Thus, material properties (mechanical strength etc.) of commodity plastics are desirably improved. With improved material properties, commodity plastics can be used as alternatives for engineering plastics or metal materials and the production cost of various polymer or metal industrial products and household products can be reduced.
Among commodity plastics, polypropylene which has relatively high mechanical strength is regarded as a promising material and the rate of increase in annual polypropylene production is maintained at a high level of several percent. Since monomer recycling of polypropylene is easy, perfect recycling is possible and thus generation of carbon dioxide and consumption of petroleum fuels such as oil can be reduced.
Patent Literatures 1, 2, and 3 (International Publication Nos. 2007/026832, 2008/108251, and 2010/084750) disclose a method for producing a high-molecular oriented crystal such as polypropylene with improved mechanical strength, the method including stretching a melt of a polymer at an elongational strain rate equal to or higher than the critical elongational strain rate to prepare an oriented melt and crystallizing the melt of a polymer while maintaining the state of the oriented melt.